Adds a large pool of document that can and should be used prior to a
release to understand effects of the new release over a real-world
scenario.
Documents are stored in an external git LFS repo under
`tests/test_docs_large` and currently it's about 11K documents gathered
from multiple PDF readers and office suite's test sets.
Documentation on how to run the tests is under
`docs/developer/TESTING.md`
